The present invention relates generally to piezoelectric transducers and, more particularly, to piezoelectric transducers utilizing two-phase, ceramic plastic composites and the method of producing such transducers.
There are certain transducer applications, such as the generation and detection of underwater acoustic signals or biomedical sensors, where it is highly desirable that the apparatus have the following properties: low density, high compliance, and high flexibility. A low density characteristic permits efficient acoustic coupling to the fluid medium and at the same time allows the transducer's buoyancy to be readily adjusted as compared to conventional ceramic piezoelectric materials. High compliancy improves the transducer's resistance to mechanical shock. It also provides high damping when the transducer operates in a passive mode. High flexibility is important when the transducer performs as a conformal detector.
There are basically two different procedures for developing a transducer material that possesses the above seemingly conflicting attributes. One involves investigating homogeneous materials. Polyvinylidene fluoride, PVF.sub.2, is an example of such a substance. It has a high compliance and flexibility and its density is low compared to conventional ceramic piezoelectrics. However, because of its low piezoelectric strain coefficient, it has only limited value as an active device. Although its high voltage sensitivity indicates good performance as an active device, when used in this manner the material must be fixed to a curved surface which can flex in response to pressure changes. This requirement introduces difficulties in the design of the transducer, since a sealed flexible mounting for the polymer is needed that will function at extreme ocean pressures and still retain sensitivity near the surface.
A second approach involves the use of a composite material where advantage is taken of the different properties of each phase. A composite made of a polymer and lead zirconate titanate ceramic, PZT, is one choice. In such a composition, the polymer phase would lower the density and permittivity and increase the elastic compliance. If an elastomer is used, the composite would be compliant and flexible. If an epoxy is used, the transducer could perform as a resonator.
In the past elastomer/PZT composites have been produced for use as flexible, low density transducers. In these cases, the procedure has been to load a polymer film with particles of the piezoelectric material. The degree of flexibility and the magnitude of the piezoelectric strain and voltage coefficients, "d" and "g", are primarily controlled by the size of the piezoelectric particles in the heavily loaded elastomer film.
In one prior art transducer, the flexible composite was fabricated using 5 to 10 .mu.m particles in a silicone rubber matrix. The longitudinal "d" values obtained in both cases were comparable to those of the piezoelectric PVF.sub.2 material but the voltage sensitivities were lower because of the high permittivity in the composites. The difficulty with this type of composite, where the piezoelectric particles are smaller in diameter than the thickness of the polymer sheet, is that low permittivity polymer layers interleave the piezoelectric particles preventing saturation poling after the composite is formed. After some poling has been achieved, the inleaved compliant polymer attenuates the piezoelectric response of the composite.
Composites of the above type have also been fabricated with much larger pizoelectric particles, up to 2.4 mm in diameter, with the particle size approaching the thickness of composite. Since the piezoelectric particles extend from electrode to electrode, near saturation poling can be achieved. The large rigid piezoelectric particles in this arrangement transmit the applied strain so that high "d" values may be attained if this parameter is measured across the particles. This form of composite also exhibits permittivities that are low compared to homogeneous PZT, thus resulting in high "g" values. However, the problem with this composite construction is that its properties are extremely position sensitive.
It will thus be appreciated that an effective composite transducer cannot be fabricated by merely intermixing a polymer and piezoelectric ceramic particles. Not only must the composite contain component phases with the correct properties but additionally the constituent materials must be coupled in a manner which optimizes these properties in the composite. The mode of interconnection of each individual phase is of major importance since this feature controls the electric flux pattern and the mechanical stress distribution. A structure with interpenetrant phases connected in all three dimensions, it can be shown, provides the optimum connectivity.
This concept has been demonstrated with a variety of small composites which are carefully hand prepared for this purpose. To date, there is no convenient process to prepare larger samples for practical use.
A welcome contribution to the art would be a piezoelectric composite, as well as a method of making the same, which provides a flexible, low density electromechanical transducer; which is utilizable in the construction of a flexible hydrophone having a high hydrostatic sensitivity; in which a polymer phase (matrix) and a piezoelectric phase (ceramic fibers) are co-joined in such a manner that each individual phase is interconnected in either two or all three orthogonal dimensions, as desired; and which can be prepared by a process which is sufficiently flexible to allow control of the composition of the ceramic and/or polymeric materials, to control the extent of incorporation of ceramic materials and to allow the deliberate introduction of various reinforcements or other additives. Such a composite and a method for its production are provided by this invention.